1. Field of the Invention
The present invention relates to a radio-frequency switching circuit and a semiconductor device, and particularly to a radio-frequency switching circuit used for, e.g., a multi-band communication device compatible with a plurality of frequency bands, and a semiconductor device having a semiconductor substrate on which the radio-frequency switching circuit is integrated.
2. Description of the Background Art
In recent years, there has been a demand for mobile communication devices such as cellar phones to be decreased in size and improve in performance. In order to meet such a demand, Antenna Switch MMIC technology has been proposed in which a field-effect transistor (hereinafter, referred to as FET) using gallium arsenide (GaAs) is used as a signal path switching element such as an antenna.
However, an antenna switch MMIC structured by FETs has a drawback that a radio-frequency characteristic thereof is deteriorated if a high-power signal is inputted when a power supply voltage is as low as, e.g., 3V. In, e.g., Japanese Laid-Open Patent Publication 9-238059, a radio-frequency switching circuit having FETs thereof connected in a multistage manner is proposed, which prevents the deterioration of the radio-frequency characteristic even if a high-power signal is inputted. Hereinafter, such a conventional radio-frequency switching circuit disclosed in the above publication will be described with reference to FIG. 18.
In FIG. 18, a conventional radio-frequency switching circuit 51 comprises: FETs 111 to 114, 121 to 124, 131 to 134, 141 to 144, 151 to 154 and 161 to 164; resistors 211 to 214, 221 to 224, 231 to 234, 241 to 244, 251 to 254, 261 to 264, 311 to 314, 321 to 324, 331 to 334, 341 to 344, 351 to 354 and 361 to 364; capacitors 400 to 406; a common terminal 500; transmission terminals 501 and 502 (Tx); reception terminals 503 to 506 (Rx); and control terminals 601 to 606.
The FETs 111 to 114 respectively have drains and sources thereof connected via the resistors 211 to 214 which are respectively in parallel with the FETs 111 to 114. The gates of the FETs 111 to 114 are connected to the control terminal 601 via the resistors 311 to 314, respectively. The FETs 111 to 114 are serially connected. The drain of the FET 111 is connected to the common terminal 500 via the capacitor 400, and the source of the FET 114 is connected to the transmission terminal 501 via the capacitor 401. The other FETs are connected in a same manner as that of the FETs 111 to 114.
When a radio-frequency signal is transmitted from the transmission terminal 501 to the common terminal 500, a high voltage (e.g., 3V) is applied to the control terminal 601, and a low voltage (e.g., 0V) is applied to the control terminals 602 to 606. This causes the FETs 111 to 114 to be in on-state, and the other FETs to be in off-state. As a result, the radio-frequency signal is transmitted from the transmission terminal 501 to the common terminal 500.
FIG. 18 shows an exemplary configuration in which four FETs are serially connected in order to withstand an input signal power of 30 dBm when a power supply voltage is 3V. This configuration allows that an inputted signal voltage is divided by the four FETs to be controlled by a control voltage of 3V. FIG. 19 shows an equivalent circuit in which a signal is transmitted from the transmission terminal 501. Here, the FETs 111 to 114 in on-state are represented by resistors, and the other FETs in off-state are represented by capacitors. As shown in FIG. 19, the number of capacitance components of the FETs in off-state increases in accordance with an increase in the number of transmission paths. The increased number of capacitance components causes a deterioration of frequency characteristic, thereby increasing insertion loss.
When a radio-frequency signal is transmitted from the common terminal 500 to the reception terminal 503, a high voltage is applied to the control terminal 603, and a low voltage is applied to the control terminals 601, 602 and 604 to 606. This causes the FETs 131 to 134 to be in on-state, and the other FETs to be in off-state. Also in this case, there is the problem of the deteriorated frequency characteristic caused by the capacitance components of the FETs in off-state.
It is expected that in the future, mobile communication devices will have functions as multi-band devices compatible with a plurality of frequency bands, and the number of transmission/reception paths therein for radio-frequency signals will increase. However, as described above, if the number of transmission/reception paths is simply increased, capacitance values of FETs in off-state (i.e., off-capacitances) become great. This causes a problem that frequency dependency becomes great and insertion loss at a radio frequency is increased.
Also, in order to obtain isolation between a transmission terminal and a reception terminal, a shunt circuit is required to be provided for each terminal as shown in an exemplary radio-frequency switching circuit 52 of FIG. 20. However, when a circuit has a configuration in which a shunt circuit is provided for each terminal, complexity of the configuration of the circuit increases in accordance with an increase in the number of transmission/reception paths. This causes a problem that a semiconductor chip increases in size.